Mysterious Heroine X Alter in her Third Ascension outfit from Fate/Grand Order. This costume is a parody of the Sith Lords from Star Wars, with her wearing a black hooded trenchcoat, black gloves, a black leotard, blue thighhighs, and black shoes. Her eyes also noticeably change color from solid yellow to just yellow around the pupils and red for the irises. post #2663437 is this ascension's official art.
This tag implicates mysterious_heroine_x_alter_(fate) (learn more).